1. Field of the Invention
This invention relates to coating compositions which comprise a polymer having at least two hydroxy functional groups and a crosslinking agent having an average of at least two pendent ester groups of the general formula: ##STR2## wherein R.sup.1 and R.sup.2 are independently hydrogen, alkyl, substituted alkyl or aryl groups; n is from about 1 to about 5; X is nitrogen, sulfur or phosphorus: and when X is either nitrogen or phosphorus, R.sup.3 is hydrogen, lower alkyl, or an aryl group; R.sup.4 is lower alkyl or an aryl group; and when X is sulfur, R.sup.4 is nothing and R.sup.3 is either hydrogen, lower alkyl or aryl.
2. Description of the Art
Methods of crosslinking hydroxy functional polymers and polymeric polyols with esters are known. For example, U.S. Pat. No. 4,423,167 teaches coating compositions formed by transesterifying a polymeric polyol with a polyester crosslinking agent having at least two beta-alkoxyester groups per molecule. Similarly, U.S. Pat. No. 4,440,612 teaches a method of coating a conductive substrate using the composition of U.S. Pat. No. 4,423,167. U.S. Pat. No. 4,489,182 teaches coating compositions formed by transesterifying a polymeric polyol with a polyester crosslinking agent having at least two gamma and/or deltahydroxyester groups per molecule. U.S. Pat. No. 4,511,447 teaches a method of coating a conductive substrate using the composition of U.S. Pat. No. 4,489,182. U.S. Pat. No. 4,423,169 teaches coating compositions formed by transesterifying a polymeric polyol with a polyester crosslinking agent having at least two beta-ester and/or gamma-ester ester groups per molecule. U.K. Patent Application No. GB 2 102 430 A teaches coating compositions formed by transesterifying a polymeric polyol with a polyester crosslinking agent having at least two ester groups containing a proximate substituent group per molecule, selected from the group consisting of beta-alkoxy, beta-carboxyalkyl, beta-amido, gamma-hydroxy, gamma-carboxyalkyl, and delta-hydroxy groups. European Patent Application No. 0 012 463 A1 teaches thermosetting resinous binders useful as coating compositions comprising a hydroxy-functional polymeric resin and a polyester crosslinking agent having at least two beta-hydroxy ester groups per molecule. U.S. Pat. No. 4,427,805 teaches thermosetting binder compositions comprising a hydroxy-functional polymeric resin and a polyester crosslinking agent having at least two beta-hydroxy ester groups per molecule. U.S. Pat. No. 4,491,611 teaches catalytically self-crosslinking resinous binders prepared by addition reaction of a primary amine to a beta-hydroxyalkyl ester of an alpha-beta-alkenylcarboxylic acid followed by reaction of this amine adduct with an epoxy resin to produce a resin having both hydroxy functionality and beta-hydroxyalkyl ester groups.
In each of the above prior art patents, the crosslinking mechanism is promoted by an activation of the reactive ester group. This is accomplished through an interaction of the ester carboxyl group with the available electron pair on the neighboring activating group, e.g., the oxygen atoms from the beta- or gamma-hydroxy or alkoxy group. The degree of activation is directly proportional to the degree of the availability of the electron pair, i.e. nucleophilicity, from the activating oxygen. The nucleophilicity of oxygen in the prior art is low compared to the nucleophilicity of nitrogen, sulfur or phosphorus, consequently, the magnitude of activation provided by oxygen is lower, and the result is the apparent need for higher crosslinking temperatures than those needed by the present invention.
The present invention relates to coating compositions formed by crosslinking a polymeric resin having at least two hydroxy functional groups with a polyester crosslinking agent having at least two ester groups per molecule of the general formula: ##STR3## wherein R.sup.1 and R.sup.2 are independently hydrogen, alkyl, substituted alkyl or aryl groups; n is from about 1 to about 5; X is nitrogen, sulfur or phosphorus; and when X is nitrogen or phosphorus, R.sup.3 is hydrogen, lower alkyl, or an aryl group; R.sup.4 is lower alkyl or an aryl group; and when X is sulfur, R.sup.4 is nothing and R.sup.3 is either hydrogen, lower alkyl or aryl.
The crosslinked resins of the present invention are excellent thermosetting coating compositions, and are particularly useful in cathodic electrodeposition processes.